Muscles grow by increasing the size of their cells, not by increasing the number of cells. In contrast, the cells of the immune system and body mainly increase by reproducing in response to infection.

These cell divisions have a limit. Telomeres, at the end of chromosomes, shorten with each division until the cell stops dividing or dies.

#Covid19 shortens Telomeres. Yes, immune systems will age.
https://mastodon.green/@fitterhappierAJ/109372994883071557